As is known, wireless telecommunications systems comprise a network infrastructure and mobile terminals. The network infrastructure generally comprises one or more interconnected operation centres and Base Radio Stations, which are organised to ensure coverage for a given area of territory and communicate with a respective operation centre. The mobile terminals connect to the network infrastructure through or more Base Radio Stations and can, for example, include mobile phones, notebook computers or palmtops with radio frequency link functions and the like.
The network infrastructure requires testing in order to check correct operation. Tests on the entire network infrastructure or on part of it can become necessary for various reasons. For example, in the design and implementation phases, it might be necessary to check the functionality of the Base Radio Station when linked to one or more terminals, the behaviour of which can be modified for the purpose of simulating fault situations or communications protocol errors. In addition, it might be necessary to check the expected behaviour of the base radio station in the presence of network loads caused by a number of simultaneously active user terminals.
To this end, test simulators are usually employed that, in practice, allow simulating the link to the network infrastructure and its utilization by one or more mobile terminals. In this way, if is possible to subject the network infrastructure or a part thereof to set operating conditions and assess the response of the entire system according to certain parameters.
A fundamental, yet at the same time very problematic aspect in the design and implementation of tests for communication systems is the possibility of accurately modelling the transmission channel, i.e. of correctly taking into consideration the effects of the transmission medium and phenomena that affect the signal between the transmission point and the reception point. One of the main problems in mobile radio communication systems derives from the fact that the mobile terminals are often in movement and therefore the properties of the radio channel change quickly. In fact, the pulse and attenuation response characteristics of the radio channel can even change thousands of times a second with wide variations in both phase and amplitude.
The properties of the radio channel and its transfer function, characterized in terms of phase and amplitude time changes as a function of time, are determined from the combination of two phenomena. A first phenomenon is linked to the presence of multipath fading, or rather to the fact that mobile terminals receive the signals transmitted from the base radio stations both directly and via reflections and therefore with different phase relations. A second phenomenon is linked to the Doppler Effect, which occurs when the mobile terminal is in movement with respect to the base radio station.
The disturbances on the radio channel are added to the effective signal, which represents transmitted sequences of symbols, and therefore renders decoding of the received message more difficult, with the possibility of introducing errors and therefore of invalidating the reception of data blocks.
To check the effects of the radio channel, channel simulators are used that process the test sequences generated by the mobile terminal simulators. To this end, the channel simulators use reference models to apply the effects due to the most common phenomena, for example, such as selective attenuations in the time and frequency domains due to the addition of a same reflected signal and interference signals originating from multipath fading.
The radio channel simulation devices currently in use have an input port and an output port and have a transfer function similar to that of a real transmission channel. A mobile terminal (or a mobile terminal simulator) is connected to the first port and a Base Radio Station is connected to the second port by means of appropriate radio frequency links.
An important limitation of known radio channel simulator devices is constituted by the possibility of only simulating one radio channel at a time. Instead, in a communication network under real conditions of use, several mobile terminals, with different movement profiles, can be simultaneously connected to the same Base Radio Station.
The complexity of telecommunications networks made using OFDM modulation, which is due both to the method of modulation and to the management of radio resources, requires verification and validation techniques based on the simulation of a radio environment with multiple, mutually independent mobile terminals. The most important functionality of a Base Radio Station increasingly resides in the management of radio resources, namely in the capacity to optimize the utilization of available bandwidth, minimizing interference and retransmission requests and maximizing achievable performance over a multiplicity of terminals. The most critical situations arise when users are distributed inside the area covered by the cell and especially in the so-called cell border area, the boundary area between adjacent cells. These situations require accurate tests under realistic conditions, which cannot be created using a sole conventional channel simulator for the entire population of terminals.